1. Field of the Invention
The present invention relates generally to power supply devices, image forming apparatuses, and voltage output methods.
2. Description of the Related Art
An electrophotographic image forming apparatus typically forms an image on a recording medium (recording paper) by forming an electrostatic latent image on an image carrier that is uniformly charged, developing the electrostatic latent image with toner to form a toner image, and transferring and fixing the obtained toner image onto the recording paper.
Meanwhile, recording paper generally has a surface roughness. Toner is less easily transferred to depressions than to projections. Accordingly, when an image is to be formed on recording paper having a highly rough surface, toner can fail to be transferred to depressions, causing the image to disadvantageously have density inconsistencies such as white spots.
To overcome this disadvantage, for example, Japanese Laid-open Patent Application No. 2012-42835 discloses the following technique. That is, a DC power supply and an AC power supply are series-connected. A voltage applied to a transfer unit to transfer an image onto recording paper is switched using a relay between a DC voltage and a DC/AC superimposed voltage depending on a degree of surface roughness of the recording paper.
However, such a conventional technique as that described above is disadvantageous in that rise time of a DC voltage that is output from series-connected power supplies is longer than a DC voltage that is output from a single DC power supply.
Under the circumstances, there is a need for power supply devices, image forming apparatuses, and voltage output methods capable of reducing voltage rise time.